Several processes have been used commercially and have been proposed to separate and recover C.sub.3.sup.+ hydrocarbons from dehydrogenation or catalytic cracking off-gas mixtures.
In an article by S. Gussow, et al., "Dehydrogenation Links LPG to More Octanes", Oil and Gas Journal, December 1980, pages 96 through 101 an absorption-stripping process is disclosed. In this process C.sub.3 through C.sub.5 hydrocarbons are absorbed into an oil along with lesser quantities of lighter components. The C.sub.3.sup.+ hydrocarbons and dissolved light impurities are then stripped from the oil in a reboiled stripping column and condensed in an overhead condenser. This process is characterized by high energy requirements, particularly to supply the fired reboiler heat. In addition, large, expensive columns and associated heat exchangers and a large fired heater are required due to the high oil circulation rate necessary for high product recovery, typically, in the 98 to 99.8 percent range.
A similar absorption-stripping process is widely used for recovery of C.sub.3 -C.sub.4 hydrocarbons from catalytic cracking unit off-gas. This process is described by J. H. Gary and G. E. Handwork in Petroleum Refining, 2nd Edition, 1984, pages 208 through 210.
In U.S. Pat. No. 4,381,418. another separation process is disclosed. In this process, a dehydrogenation process off-gas mixture is compressed and cooled to a sufficiently low temperature to condense the desired heavy hydrocarbon components along with some light impurities. Refrigeration for the process is provided primarily by cooling of the liquid hydrocarbon feedstock and subsequent mixing with recycled hydrogen, followed by revaporization of the hydrogen/hydrocarbon mixture. The high hydrogen concentration of the mixture reduces the partial pressure of the vaporizing hydrocarbons sufficiently to provide refrigeration at the required temperature levels for high product recovery, e.g. -10.degree. F. to -50.degree. F. for C.sub.4 recovery. This process requires that the feedstock hydrocarbon be dried to avoid freezing at the cold vaporization temperatures. It also requires high hydrogen recycle rates in the dehydrogenation process to achieve the low hydrocarbon partial pressures required for feedstock revaporization at suitable low temperature levels.
In U.S. Pat. No. 4,519,825, a third recovery process is disclosed. In this process, the product gas mixture is compressed, cooled and partially rectified in a dephlegmator to separate the desired heavier hydrocarbons from the bulk of the light impurities. The light gases are expanded to provide refrigeration for the process. With typical C.sub.4 dehydrogenation off-gases, this process requires no low level, i.e. below 20.degree. F., auxiliary refrigeration, but requires that the off-gas be compressed to a relatively high pressure, e.g. in the range of 350 to 550 psia, in order to provide sufficient expansion refrigeration for high product liquids recovery, e.g. 98 to 99.8+ percent. A large fraction of the C.sub.4.sup.+ hydrocarbons, e.g. more than half, is typically condensed via cooling water or air cooling in the compressor aftercooler. A small quantity of high level refrigeration, i.e. 35.degree.-65.degree. F., is necessary if the off-gas is further precooled prior to drying. With a typical lean refinery gas, this process requires that the gas be compressed to 225 psia in order to provide sufficient expansion refrigeration for high C.sub.4.sup.+ liquids recovery, e.g. 98.5 percent.
In all of the prior art processes described above, downstream fractionation of the recovered C.sub.3 to C.sub.5 hydrocarbons is usually necessary to achieve the desired product purity levels or to separate unreacted feedstock hydrocarbons for recycle or other use.
Several processes have been disclosed which utilize an absorption heat pump refrigeration cycle to provide refrigeration to separation and liquefaction processes.
In U.S. Pat. No. 4,350,571 a process and apparatus for reducing the amount of energy which must be supplied to thermally activated separation processes such as fractional distillation, distillation, dehydration or acid gas scrubbing is disclosed. The reduction is accomplished by incorporating an absorption heat pump into the process such that the absorption heat pump accepts rejected heat from, i.e. provides cooling to, the process and supplies high temperature heat back to the process. The absorption heat pump causes the necessary temperature increase through the motive power of an external heat source applied to it, in contrast to the mechanical power source required by conventional heat pumps.
In U.S. Pat. No. 3,817,046, a combination cooling process which is particularly useful for the liquefaction of natural gas is disclosed. The process employs a multi-component cooling cycle coupled to an absorption refrigeration cycle, and utilizes the waste exhaust energy from a driver for compressors in the multi-component cycle to effect heating in the absorption refrigeration cycle.